1. Field of the Invention
The present invention relates to a voltage controlled oscillator, and more particularly, to a voltage controlled oscillator including a voltage-current converter outputting current according to input voltage and a current controlled oscillator controlling frequency of an output signal according to input current.
2. Description of Related Art
One of oscillators used for a semiconductor device includes a voltage controlled oscillator. The voltage controlled oscillator is used for an output part of a PLL (Phase Locked Loop), for example. The voltage controlled oscillator varies frequency of an output signal according to input voltage. At this time, the voltage controlled oscillator varies the frequency of the output signal in a predetermined range with respect to an input voltage range. One example of the voltage controlled oscillator is shown in FIG. 8.
A voltage controlled oscillator 100 shown in FIG. 8 includes a voltage-current converter 110 and a current controlled oscillator 120. In the voltage controlled oscillator 100, the voltage-current converter 110 generates control current Is according to input voltage VC. The control current Is is input to the current controlled oscillator 120. The current controlled oscillator 120 controls frequency of an output signal FOUT according to a magnitude of the control current Is.
As stated above, in the voltage controlled oscillator, the voltage-current converter converts input voltage into current, and the frequency of the output signal FOUT is controlled based on the converted current. The voltage-current converter is also used in an application other than the voltage controlled oscillator. One example of the voltage-current converter is disclosed in Japanese Unexamined Patent Application Publication No. 2004-274569. FIG. 9 shows a circuit diagram of a voltage-current converter 200 disclosed in Japanese Unexamined Patent Application Publication No. 2004-274569. The voltage-current converter 200 generates current Ie2 by a resistor R200 based on a voltage value of input voltage Vin. The generated current Ie2 is supplied to loads RL1 and RL2 as current Ic4 and Ic5 through a current mirror formed by transistors T3 and T4.
Further, a voltage-current converter in which a differential signal functions as an input signal and current is generated based on a difference of signal levels between two signals forming the differential signal is disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-76787 and 7-183743.
However, we have now discovered that the voltage-current converter shown in FIGS. 8 and 9 cannot secure enough output current range if the input voltage range is limited. Now, the relation between the input voltage and the output current in the voltage-current converter 110 will be described with reference to FIG. 10. The voltage-current converter 110 determines an inclination of the output current and a maximum value of the output current with respect to the variation of the input voltage according to a resistance value of a resistor R100. Therefore, when the resistance value of the resistor R100 varies, there may be some cases in which the output current range cannot be secured enough with respect to the input voltage range. In the example shown in FIG. 10, when the resistance value of the resistor R100 increases, the output current is saturated in a current value equal to or smaller than the maximum value of the output current range needed to control the frequency even when the input voltage is in the input voltage range. Further, when the resistance value of the resistor R100 decreases, the output current cannot secure the output current range needed to control the frequency even when the input voltage reaches the maximum value of the input voltage range. Since the value of the output current depends on the resistor R200 also in the voltage-current converter 200 shown in FIG. 9, the problem shown in FIG. 10 occurs as well.
As disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-76787 and 7-183743, the voltage-current converter in which the differential signal functions as the input signal controls the inclination of the change of the output current by giving fluctuation reflecting the difference of the signal level of the differential signal to the reference current. However, when the value of the reference current varies, the value of the output current varies even with the constant variance rate of the output current. There is caused a problem that enough output current range cannot be secured with respect to the input voltage range if such a variation is caused when the input voltage range is limited.
When the voltage-current converter in which enough output current range with respect to the input voltage range cannot be secured is used as the voltage controlled oscillator, it is impossible to secure the enough frequency variable range with respect to the input voltage range. In the recent semiconductor device which is operated in a low voltage, the input voltage range cannot be made wide. Hence, the problem that enough output current range with respect to the input voltage range cannot be secured obviously influences the frequency variable range of the voltage controlled oscillator.